Proximity detection has been accomplished by use of magnetics, acoustics, optical and other forms of electromagnetic radiation. All active systems generate a signal which is transmitted toward an object from which it is reflected, though diminished, to the sender, where it is processed to determine the distance to the target by the time it takes for the signal to make the round trip.
Proximity detection by detecting a magnetic field has long been used and its limitations are well understood.
Acoustics have long been used for proximity detection in water. An acoustic signal is generated and directed into water toward the intended or suspected target from which it is reflected. One of the disadvantages of the acoustic system is that the acoustic signal is not easily directionally controlled and tends to scatter once released. This tends to notify a second listener of the position of the sender. Furthermore the speed of sound in air and water is relatively slow, a fact which may eliminate it from some uses where time and speed are factors, particularly in high speed operations.
One of the advantages of using light for proximity detection is that its transmission rate is approximately one million times faster than that of sound. Furthermore, it can be directed in a narrow pattern to yield high resolution and does not scatter to dissipate itself and generally announce itself. Several light beams can be directed on a target as they do not mix or interfere with one another when they cross or are otherwise brought together. In the past, it has been proposed to merely direct a beam of light on an object and measure the light reflected from its surface to provide detection. The round trip time for the light to reach the object and return is difficult to measure, since light travel 186,000 miles/second. Thus peak detection is usually used. However, this approach has some disadvantages. The contour, color, or surface texture of the object surface are unknown factors which affect light reflection. With known systems, there is no way of determining whether the magnitude of the reflected light is caused by surface condition or proximity of the object. Furthermore there are no known means of determining when the maximum return signal has been reached. Therefore, a command signal is provided upon the reflected signal reaching some predetermined threshold.
Glass optical systems tend to be large, delicate, expensive, and unsuitable for practical use in normal environments. Optical fibers, on the other hand, overcome many of the problems associated with bulky optics. They are rugged and can be used for beam forming to avoid delicate beam forming problems associated with bulk objects. Optical fibers allow substantial freedom in the location of light sources and receivers.